Recently, methods for the preparation of olefin polymers using a catalyst composed of a zirconocene compound and aluminoxane as a new type of Ziegler olefin polymerization catalysts have been proposed, for example, in Japanese Patent L-O-P Publns. Nos. 19309/1983, 35007/1985 and 221208/1986. According to these publications cited above, it is reported that ethylene polymers having a narrow molecular weight distribution and a narrow composition distribution and excellent in transparency are obtained. However, the polymers obtained by the use of such olefin polymerization catalysts as mentioned above have a narrow molecular weight distribution and are poor in processability on molding equipment, hence it is desired that the polymers shall be improved in melt properties according to the purpose for which they are used.
With the view of improving the above-mentioned methods, Japanese Patent L-O-P Publns. Nos. 35006/1985 and 35008/1985 propose a combination use of two or more kinds of metallocene compounds as the olefin polymerization catalysts, and Japanese Patent L-O-P Publns. No. 501369/1988 proposes a combination use of a metallocene compound and non-metallocene compound as the olefin polymerization catalysts. However, none of these proposals are found yet to be wholly satisfactory.
Furthermore, the polymers, particularly copolymers obtained by the use of the above-mentioned olefin polymerization catalysts are low in melting point and poor in heat resistance, hence it is desired said polymers or copolymers shall be improved in heat resistance.
On the one hand, ethylene copolymers obtained by the use of titanium based catalysts composed of a titanium compound and an organoaluminum compound are excellent in heat resistance, but have such drawback that when they are prepared so as to have a low density, the amount of their hydrocarbon solvent-soluble portion becomes large and they exhibit poor anti-block properties.